Mode-to-mode vibrational energy flow in S1 benzene. V–V resonant energy transfer, microscopic reversibility, and the role of level degeneracies

Abstract

The pattern of mode-to-mode vibrational energy flow from the level ν6′=522 cm−1 in the S1 state of benzene has been studied with OCS as a collision partner. OCS has a near resonant bending vibration ν2″=520 cm−1. V–V transfer to this bend of OCS is efficient, requiring only ten collisions. However, a T,R→V transfer from OCS adding ν16′=237 cm−1 to the ν6′ benzene fundamental is even more efficient. Since other V–T,R transfers are also fast, the V–V resonance does not dominate vibrational energy flow pattern from the initially excited benzene level. As with other added gases energy flow is governed by strong propensity rules which can be expressed simply in terms of final level degeneracies, vibration quantum changes, and energy exchange between V and T,R degrees of freedom. Measurements of V–T,R transfer rates from other S1 benzene levels with added CO have been used to demonstrate microscopic reversibility and to show the control of transfer rates by the degeneracies of the final level. OCS also quenches the S1 electronic state of benzene with a rate constant k= (1.1±0.1) ×106 Torr−1 s−1 for destruction of the S1 state with a 300 K Boltzmann distribution of vibronic levels.